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Haven't tryied it but can't see a problem with it.
You wont be able to use the mirroring 2.0 that came in cms 6 but it also has the old mirroring 1.0 so if you set that up I'm sure you'll be able to mirror between cms5 and cms6
Is it possible to do a mirroring between a cms 5 and a cms 6 site?
All I could found about this when searching was "Both sites must run EPiServer CMS 5 R1 or later.",
but does that mean both sites must run the same version of the cms also?